The use of robot arms for positioning and placing objects is well known. Generally, the arms have Z, R and .theta. motion in a conventional cylindrical coordinate system. The capability of providing straight line motion is very important in the processing of semiconductor wafers so as to allow them to be very accurately positioned at a work station where processing steps take place. The R or straight line radial movement of the end effector or mechanical hand at the end of the arm has been accomplished in a number of manners.
As one example, telescoping arms have been utilized for this purpose. In such a structure one slidable member fits within another thus allowing linear extension of the arm.
More commonly, two link arms with equal length links have been utilized for this purpose. The links are connected to each other so that the distal end of the first link is pivotally attached to the proximal end of the second link. The links utilize belt drives which are provided for coordinately rotating the second link to the first link to provide a rotation ratio, i.sub.2,1 of 2/1, and to provide a rotation ratio, i.sub.3,2, of 1/2 between the end effector and the distal link of the robotic arm. When i.sub.2,1 is equal to 2/1 and i.sub.3,2 is equal to 1/2, the result is that i.sub.31, the rotation ratio of the end effector relative to the first link, is equal to 2/1.times.1/2 or unity and radial straight line motion results. In the case of 3 link arms, such as those shown in U.S. Pat. No. 5,064,340, the rotation ratio between the third and second links is 1/1 and other ratios are as just discussed above. In this situation i.sub.21 is equal to 2/1, i.sub.3,2 is equal to 1/1 and i.sub.4,3 is equal to 1/2. This assures that i.sub.4,1 is equal to unity and radial straight line motion results.
United Kingdom Patent Application GB 2193482A, published Feb. 10, 1988 discloses a wafer handling arm which includes two unequal length links with the distal end of one link being pivotally attached to the proximal end of the other link, with the hand being integral with the distal end of the distal link and which utilizes a belt drive which is fixed to a cam to attain nearly straight line motion.
It is also known to utilize an isosceles triangle type linkage wherein two equal length links are pivoted together and a mechanical hand is pivoted to the distal end of the distal link. Pulleys and belts are utilized in such a manner that the angle between the two links changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. This linkage provides drive directly from a motor shaft to the proximal end portion of the proximal link. A belt about a stationary pulley coaxial with the motor shaft passes about a pulley at the point of pivoting of the two links to one another. Another pulley and belt arrangement provides pivoting of another pulley where the second link is pivotally connected to the mechanical hand. Radial straight line motion results.
In another apparatus a pair of isosceles triangle type linkages face one another and the mechanical hand is pivotally attached to the distal ends of both of the distal links. The proximal ends of each of the proximal links is driven in an opposite direction of rotation by a single rotating motor shaft, generally through use of appropriate gearing. What results is a "frogs leg" type of motion with each isosceles triangle type linkage serving as means for controlling the other such linkage in such a manner that the angles between the two links of each of the isosceles triangle linkages changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. The result is radial straight line motion. The frogs leg linkages have the advantage of extra strength and are particularly useful under certain conditions, particularly in vacuum environments, since they tend to require less moving parts within the vacuum chamber whereby dust is less likely to develop.
In previously mentioned U.S. Pat. No. 5,064,340, which is incorporated herein in its entirety by reference, an arm structure is disclosed comprising first, second and third longitudinally extending links each having proximal and distal end portions. The second longitudinally extending link is twice the effective length of the first link. The proximal end portion of the second link is pivotally mounted to the distal end portion of the first link about a first pivot axis. The proximal end portion of the third link is rotatably mounted about a third pivot axis to the distal end portion of the second link. An end effector is pivotally mounted to the distal end portion of the third link for rotation about a fourth pivot axis. Means is provided for coordinately rotating the first link, the second link, the third link and the end effector at a rotation ratio of the first axis to the second axis to the third axis to the fourth axis of 1:2:2:1. Again, the torque of the end effector pivot equals that of the driver. Radial straight line motion is provided.
There is a problem which is common to all of the above described radial positioning arms. This problem is that the arms must sit idly by while a workpiece is being worked upon. For example, a semiconductor wafer is picked up from a loading cassette by an end effector located at the end of the arm. The wafer is moved to a processing station and deposited. The arm moves away and sits idly by until processing at the station is completed. Once the process is completed the single end effector must move into the processing chamber, pick up the processed wafer and retract, rotate to the receiving cassette, place the processed wafer in the receiving cassette, rotate to the loading cassette, move in and pick up another wafer, retract, rotate back to the processing chamber, place the wafer and retract again to wait for the process to be finished. This is a total of eleven movements, and the time these movements take limits the throughput, i.e., the number of workpieces (e.g., wafers) which can be processed in a given time.
U.S. Pat. No. 5,007,784, which is incorporated herein in its entirety by reference, has addressed the above problem by providing a robotic arm which comprises an end effector structure which has a central portion and two substantially oppositely extending hands each capable of picking up a workpiece. The central portion is centrally pivotally mounted to the distal end portion of a distalmost of the links. The links, end effector structure and static structure are constructed to allow the robotic arm to reverse across the pivot axis of the proximal end portion of the proximalmost of the links. Radial drive means drives the links in a manner such that the pivot axis of the central portion of the end effector structure moves only substantially linearly radially along a straight line passing through and perpendicular to the pivot axis of the proximal end portion of the proximalmost of the links and to the pivot axis of the central portion of the end effector structure. The end effector structure is maintained at a selected angle to the line. Rotational drive means is also present.
Another and more recent development allows the attainment of radial straight line motion using robotic arms whose links are not so closely defined in terms of relative lengths. Such a robotic arm is disclosed in copending application Ser. No. 08/432,682, filed May 2, 1995, which is incorporated herein by reference.
A former commercial device is similar to the device of U.S. Pat. No. 5,007,784. However, in the former commercial device the end effector structure has two hands which extend from the pivot axis of the end effector at substantially right angles to one another. The end effector can assume one of two different rotational positions about its pivot axis. The two positions are substantially the right angle apart and are such that either hand can be positioned so as to move in a straight line with the distal end of the distalmost of the links.
A very important problem which exists with present day robotic arm mechanisms is that they can only follow a radial straight line (R) path or a circular (.theta.) path in the R, .theta. plane from one point another. Accordingly, if there is an object to be picked up and moved which is located in a cassette, e.g., a wafer cassette, or at a work station, the arm must first be extended radially into the cassette or work station where it picks up the object, generally by application of a vacuum, then withdrawn radially from the cassette or work station, then rotated to opposite another cassette or work station and then advanced radially into the other cassette or work station where it deposits the object. This provides an overall jerky stop/start motion which can lead to damage to the apparatus over many thousands of operations and which also vibrates the object being moved with possible deleterious effects on product yield. Also, if there is an obstacle which protrudes into that plane of operation of the arm as may occur in a semiconductor processing operation, an inefficient path must be followed to anything hidden behind or shadowed by that obstacle, namely, a straight line path must be followed to withdraw the arm radially inwardly beyond the obstacle and then radial motion must be imparted to move the end effector of the arm past the obstacle followed by radial outward motion to the desired work station. The ability to follow a curved path would be desirable in that it would allow coordinated R and .theta. motion and thereby faster operation of the robotic arm mechanism.
Another very important problem which arises in such robotic arm systems is related to loading in cassettes filled with wafers near a robotic arm. If more than one cassette is used which may be accessed with the arm it is necessary that each cassette be arranged with its longitudinal axis (the direction from which it is loaded and unloaded) passing through the center of rotation of the robot arm. This restriction exists because the arms can be extended in a straight line in only a radial direction. This does not allow for the use of a straight conveyor belt to bring in the cassettes whereby the process must be carried out by hand, an inherently inefficient way to operate. If it were possible to remove wafers from loaded cassettes which were traveling along a straight line path, namely, via a conveyor belt (in which instance their longitudinal axes would not all pass through the center of rotation of the robot arm) the speed of the overall operation could be significantly increased. This would directly lead to increased throughput and thereby to an increased profit/lower cost operation. What is done at present is to move the robot from between one cluster of radially aligned cassettes and work stations to another using a track system; a far less desirable way to operate.
A particular problem which occurs with flat display panels is that they are often present in cassettes with a certain degree of angular and linear misalignment. Such panels must be aligned properly at a work station. With conventional robotic arms this must be accomplished by placing the panel on a chuck, which has sensors, e.g., CCD sensors, which detect the misalignment, using the chuck to rotate the panel, and then picking it up and transporting it to the work station in proper alignment. This is so since with conventional robotic arms the arm cannot be rotated with respect to the panel (or wafer) coordinate frame which does not coincide (due to the position and angular misalignment) with the coordinate frame of the end effector. The conventional arms can move only along the longitudinal axis of the end effector, and to rotate it so it cannot compensate for the misalignment without intermediately being dropped onto a pin or the like. It would be highly desirable if the desired alignment could be attained without the intermediate use of such a chuck or pin. In a similar manner, it would be highly desirable if it was possible to align wafers about their geometric centers without utilizing such a chuck or pin.
Another problem which is common with both wafers and panels which are loaded from cassettes or picked up at work stations is that the Z axis of the robotic arm may not be completely parallel to the axis of the cassette, etc. due to alignment errors. The relative tilt may be in any direction and is usually only a few degrees. When this misalignment is present the arm cannot properly approach and pick up the wafers/panels with the end effector properly aligned whereby the exact positioning of the wafers/panels is not fully controllable. Previously filed U.S. patent application Ser. No. 08/661,292, filed Jun. 13, 1996, which is incorporated herein in its entirety, discloses a robotic arm having a universally tiltable Z-axis. The structure set forth in the application, however, does not have the ability to rotate at or near the wrist of the robotic arm and thus is not sufficient in and of itself to provide the necessary compensation for a misalignment of the nature described. It would be desirable if the misalignment could be fully corrected and the wafers/panels properly aligned without the intermediate use of a chuck or pin as discussed above.
With prior art robotic arms for processing semi-conductor wafers and flat panels the cassettes and work stations must generally be aligned so as to be entered by the end effectors of tie arms with the end effector moving radially in a straight line. It is not, for example, possible to enter the cassettes or workstations if their longitudinal axes are not parallel to the Z axis. Yet, since space is at a premium in a wafer or flat panel processing operation it would be highly desirable to be able to so arrange cassettes and/or workstations.